Mechanical metal bearings that have been generally used for bridges, include metal rocker bearings and metal plate bearings which are designed to release horizontal displacement by a rolling or sliding action as well as to support a heavy vertical load on a small contact area of metal bearings. These types of bearings have two serious problems:
1. One problem is that, as time passes, corroding action at the metal contact area causes gradual loss of the rolling or sliding function of the bearing units, unless these are properly cleaned and maintained. PA1 2. Another problem is that these conventional metal bearings are vulnerable to reciprocal seismic force because they do not have proper shock absorbing systems to release seismic impact. PA1 a. They require more bearing contact area due to having a lesser bearing capacity than mechanical metal bearings. PA1 b. They might result in shear failure by a seismic reciprocal force crossing beyond the limit of the shear rigidity of synthetic rubber.
Synthetic rubber bearings reinforced by metal plates have been more popular for use as bearings than mechanical metal bearings, because they do not seem to have a serious corroding problem, as mechanical metal bearings do. They are capable of absorbing seismic impact to a certain level due to the flexibility of synthetic rubber itself. However, synthetic rubber bearings have some problems which are described below: